This invention relates to a fractionation operation for separating propylene from propane. In one of its aspects this invention relates to a gas cracking and separating operation in which some of the separated products such as propane and ethane are recycled to the gas cracking operation. In another if its aspects this invention relates to the control of a fractionation operation by manipulation of feedstock to the fractionation. In still another aspect of this invention it relates to the addition of a component to the feedstock of a fractionation operation in response to the analyzed content of a specific compound in the fractionation process.
One of the problems encountered in a process of gas cracking of combination feedstocks of ethane, butane, and propane followed by compression and gas separation of the high pressure gas stream by a seriatim fractionation to separate the components of the gas stream by boiling point difference with recycle of some of the product streams to the gas cracking operation can be the buildup of unwanted materials in the various fractionation columns and product streams. Some of these unwanted components can actually prove to be of danger in the fractionation process or if recycled into the gas cracking operation. These unwanted components often provide problems in reaching specification limitations of product streams. In the fractionation to separate propylene from propane in a stream that has been recovered from de-ethanized and de-butanized gas cracking product the concentration of methylacetylene and propadiene in the bottoms from the fractionation increases proportionally as the concentration of propane in the feedstock decreases. Particularly when the propane stream is recycled to the gas cracking operation the high content of acetylenes is potentially explosively hazardous. The high content of methylacetylene and propadiene also makes the separation of propylene from the feed mixture more difficult without increasing the amount of methylacetylene and propadiene in the propylene product. Generally, the propylene product has a low specification limit for content of these compounds.
It has now been found that the controlled injection of propane from an outside source into the feedstream to increase the propane content in the separation column provides the necessary propane content in the bottoms product sufficiently to dilute methylacetylene and propadiene until the problems are eliminated. The present invention provides a method for controlling the injection of propane into the propylene/propane separation column feedstream.
It is therefore an object of this invention to provide a method for controlling the propane in the inlet to a propylene/propane separation column. It is also an object of this invention to inject a controlled amount of propane into the inlet stream of a propylene/propane separation column. It is another object of this invention to alleviate the buildup of methylacetylene and propadiene in the bottoms stream of a propylene/propane separation column. It is still another object of this invention in a gas cracking and separation system to provide a propane recycle stream to be used as at least a part of the feedstock in the gas cracking operation.
Other aspects, objects and the various advantages of this invention will become apparent upon reading the specification, studying the accompanying drawing, and reading the appended claims.